Objective:Dietary supplementation with marine oils attenuates the responses to noradrenaline and angiotensin II in human forearm resistance arteries. The mechanisms underlying these effects were the subject of the present study.Methods:Twenty-two normal male adults were allocated to one of three groups. The first group (n = 11) received 10 g/day marine oil capsules (maxEPA) for 28 days. The second group (n = 7) received maxEPA plus 25 mg indomethacin three times a day on days 28 and 29. The third group (n = 4) received 10 g/day mixed-oil placebo capsules for 28 days, plus indomethacin on days 28 and 29 as in group 2. Forearm venous occlusion plethysmography was performed before and immediately after each treatment period.Results:Responses to acetylcholine, sodium nitroprusside or reactive hyperaemia (area under the time-response curve: pre-maxEPA 14 850 ± 3502, post-maxEPA 17 118 ± 4576 units) were unaffected by maxEPA. The suppressive effect of maxEPA on responses to noradrenaline and angiotensin II (from group 1) was no longer apparent in the group receiving indomethacin in addition to maxEPA. Indomethacin, in subjects on placebo capsules, had no effect on the responses to either agonist.Conclusion:We conclude that the suppressive effects of maxEPA result from alterations toin vivoprostanoid profiles.